Sensitivity correction method for dose monitoring device and particle beam therapy system

ABSTRACT

In a particle beam therapy system which scans a particle beam and irradiates the particle beam to an irradiation position of an irradiation subject and has a dose monitoring device for measuring a dose of the particle beam and an ionization chamber smaller than the dose monitoring device, the ionization chamber measuring a dose of a particle beam passing through the dose monitoring device, the dose of the particle beam irradiated by the dose monitoring device is measured; the dose of the particle beam passing through the dose monitoring device is measured by the small ionization chamber; and a correction coefficient of the dose measured by the dose monitoring device corresponding to the irradiation position is found based on the dose of the particle beam measured by the small ionization chamber.

TECHNICAL FIELD

The present invention relates to particle beam therapy systems whichperform scanning irradiation and, more particularly, relates to asensitivity correction method for a dose monitoring device of a particlebeam for use in an irradiation apparatus of its system and a particlebeam therapy system.

BACKGROUND ART

A dose monitoring device is known as so-called an ionization chamber.For example, in Patent Document 1, there is disclosed a monitoringdevice for measuring a particle beam in which a collector electrodewhich is formed by adhering metal to a resin plate by vapor depositionor plating and a high voltage electrode are arranged in face-to-facerelation by making insulation plates intervene in order to improvestrain of the collector electrode.

In Patent Document 2, there is disclosed a radiation dose monitor inwhich a high voltage electrode and a collector electrode are supportedby insulation support bodies disposed with an interval in order toprevent ionization current from changing due to deflection.

Further, in Patent Document 3, there is disclosed a transmission typedosimeter in which a dose of radiation passing through the dosimeter ismeasured and the measured dose is corrected on the basis of the amountof deformation of a container of the dosimeter due to atmosphericpressure.

RELATED ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Unexamined Patent Publication No.    H1-98985-   Patent Document 2: Japanese Unexamined Patent Publication No.    H1-210890-   Patent Document 3: Japanese Unexamined Patent Publication No.    2010-54309

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the conventional dose monitoring device (dose monitor),solution for countermeasures in the case of the occurrence of deflectionthat is inevitable for the collector electrode are not implemented. Inthe dose monitoring device, the deflection of the collector electrode issmall and the influence due to the deflection can be negligible in thecase where a transmission window of a particle beam is small in borediameter; however, with an increase in the bore diameter, the influencedue to the deflection of the collector electrode cannot be negligibleand it causes a problem that deteriorates measurement accuracy of thedose.

In view of the aforementioned problem, the present invention is toprovide a sensitivity correction method for a dose monitoring device anda particle beam therapy system, in each of which a correctioncoefficient of a dose measured by a dose monitoring device correspondingto an irradiation position of an irradiation subject is found andsensitivity of the dose monitoring device is corrected againstdeterioration in measurement accuracy of the dose due to deflection ofan electrode.

Means for Solving the Problems

According to the present invention, there is provided a sensitivitycorrection method for a dose monitoring device in a particle beamtherapy system which scans a particle beam and irradiates the particlebeam to an irradiation position of an irradiation subject, the particlebeam therapy system including: a dose monitoring device which measures adose of the particle beam; and an ionization chamber smaller than thedose monitoring device, the ionization chamber measuring a dose of aparticle beam passing through the dose monitoring device. Thesensitivity correction method includes the steps of: measuring the doseof the particle beam irradiated by the dose monitoring device; measuringthe dose of the particle beam passing through the dose monitoring deviceby the small ionization chamber; and finding a correction coefficient ofthe dose measured by the dose monitoring device corresponding to theirradiation position based on the dose of the particle beam measured bythe small ionization chamber.

According to the present invention, there is provided a particle beamtherapy system which scans a particle beam and irradiates the particlebeam to an irradiation position of an irradiation subject, the particlebeam therapy system including: a dose monitoring device which measures adose of the particle beam; an ionization chamber smaller than the dosemonitoring device, the ionization chamber measuring a dose of a particlebeam passing through the dose monitoring device; and a calculation unitwhich finds a correction coefficient of the dose measured by the dosemonitoring device corresponding to the irradiation position based on thedose of the particle beam measured by the small ionization chamber, fromthe dose of the irradiated particle beam measured by the dose monitoringdevice, the irradiation position, and the dose of the particle beampassing through the dose monitoring device measured by the smallionization chamber. The irradiation dose is adjusted based on thecorrection coefficient.

Advantageous Effect of the Invention

According to the sensitivity correction method for the dose monitoringdevice and the particle beam therapy system of the present invention,the correction coefficient of the dose measured by the dose monitoringdevice corresponding to the irradiation position of an irradiationsubject is found and sensitivity of the dose monitoring device iscorrected against deterioration in measurement accuracy of the dose dueto deflection of an electrode; whereby, the dose monitoring device canperform highly accurate dose measurement in an irradiation position ofan irradiation subject even in a relatively large irradiation fieldnecessary for scanning irradiation.

Objects, features, aspects, and advantageous effects other than theforegoing of the present invention will become more apparent from thefollowing detailed description of the present invention for referring tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic whole configuration view showing a particle beamtherapy system in which a dose monitoring device according to Embodiment1 of the present invention is located;

FIG. 2 is a configuration view showing an irradiation apparatus of theparticle beam therapy system in which the dose monitoring deviceaccording to Embodiment 1 is located;

FIG. 3 is a configuration view showing the dose monitoring deviceaccording to Embodiment 1 along with a view for explaining itsoperation;

FIG. 4 is a view for explaining deflection of electrodes of the dosemonitoring device and deterioration in measurement of the strength of aparticle beam;

FIG. 5 is a view for explaining a sensitivity correction method for thedose monitoring device in Embodiment 1;

FIG. 6 is a view for explaining a sensitivity correction method for adose monitoring device in Embodiment 3;

FIG. 7 is a block diagram showing a particle beam therapy system using asensitivity correction method for a dose monitoring device in Embodiment4; and

FIG. 8 is a block diagram showing a particle beam therapy system using asensitivity correction method for a dose monitoring device in Embodiment5.

FIG. 9 is a view for explaining a sensitivity correction method for adose monitoring device in Embodiment 6.

MODE FOR CARRYING OUT THE INVENTION Embodiment 1

FIG. 1 is a schematic whole configuration view showing a particle beamtherapy system in which a dose monitoring device according to Embodiment1 of the present invention is located. FIG. 2 is a configuration viewshowing an irradiation apparatus of the particle beam therapy system inwhich the dose monitoring device according to Embodiment 1 is located.In FIG. 1, a charged particle beam (particle beam) generated by aninjector 16 and accelerated at a former stage is made incident to anaccelerator (synchrotron) 14 and is accelerated to necessary beamenergy. The particle beam accelerated to necessary beam energy isemitted from an outgoing deflector 17 to a beam transport apparatus 15and then reaches an irradiation apparatus 18 to be irradiated to anirradiation position of an irradiation subject. The beam transportapparatus 15 has a focusing electromagnet 13 and a bending electromagnet12. A part of the beam transport apparatus 15 and the irradiationapparatus 18 are mounted on a rotating gantry 19, and an irradiationdirection of the irradiation apparatus 18 can be changed by the rotationof the rotating gantry 19 (shown by an arrow in the drawing).

FIG. 2 is an enlarged configuration view of the irradiation apparatus 18and a charged particle beam is introduced to the irradiation apparatus18 via the bending electromagnet 12 of the beam transport apparatus 15.The charged particle beam is scanned by an X direction scanningelectromagnet 20 to be controlled by a command value for X directionscanning (the amount of excitation) and a Y direction scanningelectromagnet 21 to be controlled by a command value for Y directionscanning (the amount of excitation). The scanned charged particle beamis emitted from a beam taking-out window 25 which takes out the beam viaa duct expansion and contraction unit (bellows) 22 for expanding andcontracting ducts and vacuum ducts 23,24. FIG. 2 shows conditions inwhich particle a beam 35 is scanned in a Y axial direction. Arrows XYZshow XYZ axial directions.

The particle beam therapy system which achieves scanning irradiation isdifferent from normal irradiation and layer-stack irradiation in that adose is controlled with respect to a spot irradiated on an XY flatsurface and therefore a dose monitoring device 26 of a particle beam anda position monitor 11 which measures a position of the particle beam arearranged in a lower stream of the beam taking-out window 25. The dosemonitoring device 26 and the position monitor 11 are arranged at a rightangle to an axis of a traveling direction of the particle beam.Incidentally, the same reference numerals as those shown in respectivedrawings represent the same or corresponding elements.

FIG. 3 is a configuration view showing the dose monitoring device(ionization chamber) according to Embodiment 1 along with a view forexplaining its operation. The dose monitoring device 26 has a collectorelectrode 27 and a high voltage electrode 28, each of which is retainedby electrode supports 10. The collector electrode 27 and the highvoltage electrode 28 are made of metal sheet, such as aluminum. A powersource 29 is connected between the collector electrode 27 and the highvoltage electrode 28. The collector electrode 27 is, for example, 0 V;and the high voltage electrode 28 is, for example, −1 kV or −3 kV. Anammeter 31 is connected between the collector electrode 27 and the powersource 29. The dose monitoring device 26 is covered by a container 32and an insulator 33 intervenes between the collector electrode 27 andthe high voltage electrode 28 to provide and electrical insulation andmechanical support. The dose monitoring device 26 is, for example, anopen-air ionization chamber and has a transmission window 34 of theparticle beam on each of both surfaces of parallel flat plate shape.

In FIG. 3, when the particle beam 35 passes through in the dosemonitoring device 26 filled with a gaseous substance (for example, air),the particle beam 35 impinges on molecules of the gaseous substance andthe gaseous substance is ionized. When an electric field is appliedbetween the collector electrode 27 and the high voltage electrode 28using the high voltage power source 29, an ionized electron e⁻ iscollected on the collector electrode 27 and an ion i⁺ is collected onthe high voltage electrode 28. The number of the electrons e⁻ and ionsi⁺ to be ionized is proportional to the strength of the particle beampassing through. The strength of the particle beam is measured by acurrent measured by the ammeter 31.

By the way, in the particle beam therapy system which achieves thescanning irradiation, the spot size of the particle beam preferablyreduces an increase due to scattering. Therefore, in order to reduce theinfluence of the scattering in the dose monitoring device 26, the dosemonitoring device 26 is located at a position near the lowermost streamof the irradiation apparatus 18 in FIG. 2. The position is near thelowermost stream and therefore an irradiation field of the particle beamof the irradiation apparatus 18 increases. In order to measure theparticle beam of the large irradiation field, the dose monitoring device26 increases in measurement effective area. If the measurement effectivearea increases, the collector electrode 27 is not completely parallel tothe high voltage electrode 28 in the whole region; and accordingly,nonnegligible deflection is generated in the measurement of the strengthof the particle beam.

FIG. 4 is a view for explaining deflection of the electrodes of the dosemonitoring device 26 and deterioration in measurement of the strength ofthe particle beam. The collector electrode 27 and the high voltageelectrode 28 generate deflection by electromagnetic pressure betweenboth electrodes and gravity. Dashed lines show a parallel state wheredeflection is not present in both electrodes 27,28. The electrodes 27,28deflect; and accordingly, variation is generated in the distance betweenboth electrodes 27,28 at a position (position on an XY flat surface) inthe measurement effective area of both electrodes 27,28, for example, atthe center and a peripheral portion in addition to an irradiation angleof the particle beam, and the variation is assumed to be generated inthe amount of gaseous substances between both electrodes 27,28.Therefore, measured values are different at an irradiation positionwithin the measurement effective area with respect to the same strengthof the particle beam and thus measurement accuracy of the strength ofthe particle beam deteriorates.

FIG. 5 is a view for explaining a sensitivity correction method for thedose monitoring device in Embodiment 1. The flat surface (XY surface) ofthe dose monitoring device 26 is formed with the transmission window 34of the particle beam in its central portion and is disposed inperpendicular to the axis (Z axis) of the traveling direction of theparticle beam. The position monitor 11 is disposed closely to the lowerstream of the dose monitoring device 26, and its flat surface (XYsurface) is disposed in perpendicular to the axis (Z axis) of thetraveling direction of the particle beam. Incidentally, the location ofthe position monitor 11 may be permissible if the location is a positionwhere an irradiation position of an irradiation subject can be measured(converted and measured), and the position monitor 11 may be disposed onthe upper stream side of the dose monitoring device 26 or may beseparately disposed from the dose monitoring device 26.

A small parallel flat plate type ionization chamber 38 is disposed in alower stream of the dose monitoring device 26 and the position monitor11. The small parallel flat plate type ionization chamber 38 is suitablefor measurement in which a transmission window of a particle beam issmall in bore diameter and is narrow range; and the deflection of theelectrodes can be negligible and highly accurate measurement can beperformed with respect to the strength of the particle beam. Forexample, the Bragg Peak Chamber (trade name), which is commerciallyavailable as one type of IONIZING RADIATION by PTW, is suitable as thesmall parallel flat plate type ionization chamber 38; a flat platesurface provided with a transmission window of a particle beam is, forexample, approximately a diameter of 80 mm which is sufficiently largeras compared to, for example, a beam size of 1σ=5 mm; and most particlescan be made pass through the flat plate surface. The configuration ofthe Bragg Peak Chamber is such that the dose monitoring device 26 (thetransmission window in the XY flat surface, that is, the irradiationfield is, for example, a size of 400 mm×300 mm) is reduced in size andthe configuration is similar thereto. The small parallel flat plate typeionization chamber 38 is disposed at an irradiation position of theparticle beam during sensitivity correction of the dose monitoringdevice 26; its position may be changed on a predetermined XY flatsurface (flat surface perpendicular to the Z axis of the travelingdirection of the particle beam) during the sensitivity correction; andthe small parallel flat plate type ionization chamber 38 is removedafter the sensitivity correction.

The sensitivity correction method for the dose monitoring device 26 isperformed as follows. For example, on the basis of an irradiationposition to be set based on energy of a particle beam and the amount ofexcitation of X,Y direction scanning electromagnets commanded by usingan irradiation position setting apparatus in which a treatment planningsystem has, the particle beam is scanned and the particle beam isirradiated to the irradiation position of an irradiation subject. In thearrangement shown in FIG. 5, first, the particle beam 35 is irradiatedon the axis (Z axis) of the traveling direction. The particle beam 35passes through the original point (x=0, y=0) on the XY flat surface ofthe dose monitoring device 26 and the original point (x=0, y=0) on theXY flat surface of the position monitor 11; and then, the particle beam35 reaches the small parallel flat plate type ionization chamber 38disposed at the original point (x=0, y=0) on the XY flat surface of theirradiation position of the irradiation subject. At this time, as far asthe dose monitoring device 26 is concerned, current corresponding to thestrength of the particle beam is obtained by the ammeter 31 and thecurrent is converted to a count value Do,o corresponding to theirradiation position of the irradiation subject. As far as the positionmonitor 11 is concerned, the irradiation position (0,0) on the XY flatsurface of the irradiation position of the irradiation subject ismeasured (converted and measured). As far as the small parallel flatplate type ionization chamber 38 is concerned, current corresponding tothe strength of the particle beam is obtained by an ammeter (not shownin the drawing) and the current is converted to electric charge Co,o ofthe small parallel flat plate type ionization chamber 38 correspondingto the irradiation position [irradiation position (0,0)] of theirradiation subject. In this case, the ionization chamber changes insensitivity due to amass of gas which is present between the electrodes;and therefore, sensitivity correction needs to be performed in responseto internal pressure and temperature. However, both of the dosemonitoring device 26 and the small parallel flat plate type ionizationchamber 38 use an open-air type; and accordingly, the influences of thetemperature and atmospheric pressure are balanced out and thus thecorrection does not need to be performed and it becomes possible tocalculate from only the obtained amount of electric charge.

The strength of the particle beam (electric charge) measured by thesmall parallel flat plate type ionization chamber 38 is highly accurate;and therefore, the measured value is set as a standard and a calibrationcoefficient (correction coefficient) ao,o corresponding to theirradiation position (0,0) of the dose monitoring device 26 iscalculated using Formula (1).Cx,y=ax,y·Dx,y  (1)where, Dx,y: a count value of a dose monitoring device corresponding toan irradiation position (x,y) of an irradiation subject;

ax,y; a calibration coefficient of the dose monitoring devicecorresponding to the irradiation position (x,y) of the irradiationsubject; and

Cx,y; an electric charge of the small parallel flat plate typeionization chamber corresponding to the irradiation position (x,y) ofthe irradiation subject.

Incidentally, the irradiation position (x,y) of the irradiation subjectshows a position on the XY flat surface of the irradiation position.ao,o=Co,o/Do,o  (2)

Next, the irradiation position of the irradiation subject is changed,the irradiation position of the particle beam on the XY flat surface ofthe dose monitoring device 26 is changed, and the small parallel flatplate type ionization chamber 38 is also changed to a position where theparticle beam is irradiated on a predetermined XY flat surface. In thiscase, the irradiation position (x,y) on the XY flat surface of theirradiation position of the irradiation subject is measured (convertedand measured) by the position monitor 11. The count value Dx,y ismeasured by the dose monitoring device 26 and the electric charge Cx,yis measured by the small parallel flat plate type ionization chamber 38.These measured values are substituted for Formula (1), and thecalibration coefficient (correction coefficient) ax,y at this time isobtained by Formula (3).ax,y=Cx,y/Dx,y  (3)

Further, the irradiation position of the irradiation subject is changed,the irradiation position of the particle beam on the XY flat surface ofthe dose monitoring device 26 is changed to other position, and thesmall parallel flat plate type ionization chamber 38 is also changed toother position where the particle beam is irradiated on a predeterminedXY flat surface; and accordingly, the calibration coefficient ax, y atother irradiation position of the dose monitoring device 26 can beobtained by Formula (3) in a similar way.

In this way, the calibration coefficient (correction coefficient) of thedose measured by the dose monitoring device corresponding to theirradiation position of the irradiation subject can be found based onthe dose of the particle beam measured by the small parallel flat platetype ionization chamber 38.

Furthermore, when the calibration coefficient ao,o at the position (0,0)of the irradiation position of the irradiation subject is set as astandard, a correction coefficient Ax,y at other irradiation position(x,y) of the irradiation subject can be found by Formula (4) as a ratiowith respect to a reference position (0,0).Ax,y=ax,y/ao,o  (4)

In this way, the calibration coefficient (correction coefficient) of thedose monitoring device in the case where the irradiation position of theirradiation subject of the particle beam is on the axis of the travelingdirection of the particle beam is set as a standard, and a correctioncoefficient of the dose monitoring device in the case where theirradiation position of the irradiation subject of the particle beam isdifferent from the axis of the traveling direction of the particle beammay be found by a ratio with the calibration coefficient set as thestandard.

Incidentally, block diagrams of the particle beam therapy system usingthe sensitivity correction method for the dose monitoring deviceaccording to Embodiment 1 are shown in FIG. 7 and FIG. 8.

Embodiment 2

In Embodiment 1, the irradiation position setting apparatus is provided,and on the basis of the irradiation position to be set based on theenergy of the particle beam and the amount of excitation of the X, Ydirection scanning electromagnets commanded by the irradiation positionsetting apparatus, the particle beam is scanned and the particle beam isirradiated to the irradiation position of the irradiation subject.However, in a similar way, a treatment planning system is provided, andon the basis of an irradiation position of an irradiation subjectplanned by the treatment planning system, a particle beam is scanned andthe particle beam may be irradiated to the irradiation position of theirradiation subject. Incidentally, block diagrams of a particle beamtherapy system using a sensitivity correction method for a dosemonitoring device according to Embodiment 2 are shown in FIG. 7 and FIG.8.

Furthermore, as for confirmation of the irradiation position of theirradiation subject, as described above, an irradiation position (x,y)on an XY flat surface of the irradiation position of the irradiationsubject may be measured (measured and converted) and confirmed using aposition monitor 11.

Embodiment 3

FIG. 6 is a view for explaining a sensitivity correction method for adose monitoring device in Embodiment 3. A correction coefficient whichcorrects sensitivity of a dose monitoring device 26 is preferably foundin a state near actual irradiation of a particle beam therapy system. InFIG. 6, as in Embodiment 1, a dose monitoring device 26, a positionmonitor 11, and a small parallel flat plate type ionization chamber 38are provided. In Embodiment 3, a water phantom 39 is disposed at aposition where an angle of a rotating gantry 19 (FIG. 1) is 0 degreesand an isocenter 40 is included; and the small parallel flat plate typeionization chamber 38 is disposed on an XY flat surface 36 whichincludes the isocenter 40 and is perpendicular to an axis (Z axis) of atraveling direction of a particle beam and the ionization chamber 38 ismoved to an irradiation position. The correction coefficient of the dosemonitoring device 26 can be found as in Embodiment 1, 2. If thecorrection coefficient of the dose monitoring device 26 has been found,the water phantom 39 and the small parallel flat plate type ionizationchamber 38 are removed before actual irradiation of the particle beamtherapy system. The correction coefficient of the dose monitoring device26 can be found during adjustment of the particle beam therapy systemand, further, the correction coefficient may be periodically found.

Embodiment 4

FIG. 7 is a block diagram showing a particle beam therapy system using asensitivity correction method for a dose monitoring device in Embodiment4. A dose monitoring device 26 measures a dose of an irradiated particlebeam and obtains a current corresponding to the strength of the dose.The current is converted to frequency corresponding to the current by anI/F converter 41, the converted frequency is converted to a count value(D) corresponding to the frequency by a counter 42, and the convertedcount value (D) is introduced to a calculation unit 44. A positionsensor 11 measures (converts and measures) an irradiation position (x,y)of an irradiation subject. The irradiation position (x,y) of theirradiation subject of the particle beam can be specified by theirradiation position planned by the treatment planning system describedin Embodiment 2 and is introduced to the calculation unit 44. A smallparallel flat plate type ionization chamber 38 measures the dose of theparticle beam passing through the dose monitoring device 26 and obtainsthe current corresponding to the strength of the dose; and an electrometer 43 obtains electric charge (C) corresponding to the current andthe electric charge (C) is introduced to the calculation unit 44.Incidentally, the irradiation position (x,y) of the irradiation subjectof the particle beam may use the irradiation position (x,y) of theirradiation subject converted and measured by the position sensor 11.

During adjustment of the particle beam therapy system, the particle beamtherapy system is started up and, first, a particle beam 35 isirradiated on an axis (Z axis) of a traveling direction. The particlebeam 35 passes through the original point (x=0, y=0) on the XY flatsurface of the dose monitoring device 26 and the original point (x=0,y=0) on the XY flat surface of the position monitor 11; and then, theparticle beam 35 reaches the small parallel flat plate type ionizationchamber 38 disposed at the original point (x=0, y=0) on a predeterminedXY flat surface. At this time, as far as the dose monitoring device 26is concerned, current corresponding to the strength of the particle beamis converted to frequency by the I/F converter and then counted by thecounter 42; and accordingly, a count value Do,o corresponding to anirradiation position (0,0) of an irradiation subject is measured.(Incidentally, the I/F converter 41 and the counter 42 are ordinarilyprovided as a part of the dose monitoring device 26.) As far as theposition monitor 11 is concerned, the irradiation position (0,0) on theXY flat surface serving as the irradiation position of the irradiationsubject is measured. The irradiation position (0,0) of the irradiationsubject of the particle beam can be specified by the irradiationposition planned by the treatment planning system. As far as the smallparallel flat plate type ionization chamber 38 is concerned, currentcorresponding to the strength of the particle beam is obtained from theelectro meter 43 and the obtained current is converted to electriccharge Co,o of the small parallel flat plate type ionization chamber 38corresponding to the irradiation position (0,0) of the irradiationsubject. Accordingly, the calculation unit 44 obtains a calibrationcoefficient of the dose monitoring device 26 corresponding to theirradiation position (0,0) of the irradiation subject of the particlebeam as follows:ao,o=Co,o/Do,o

Next, the irradiation position of the irradiation subject is changed,the irradiation position of the particle beam on the XY flat surface ofthe dose monitoring device 26 is changed, and the small parallel flatplate type ionization chamber 38 is also changed to a position where theparticle beam is irradiated on the predetermined XY flat surface. Inthis case, in a similar way, the irradiation position of the irradiationsubject is measured by the position monitor 11. The irradiation position(x,y) of the irradiation subject of the particle beam can be specifiedby the irradiation position planned by the treatment planning system.The count value Dx,y is obtained by the dose monitoring device 26 andelectric charge Cx,y is obtained by the small parallel flat plate typeionization chamber 38. Accordingly, the calculation unit 44 obtains acalibration coefficient of the dose monitoring device 26 correspondingto the irradiation position (x,y) of the irradiation subject of theparticle beam as follows:ax,y=Cx,y/Dx,y

Furthermore, the irradiation position of the irradiation subject ischanged to other position, the irradiation position of the particle beamon the XY flat surface of the dose monitoring device 26 is changed toother position, and the small parallel flat plate type ionizationchamber 38 is also changed to other position where the particle beam isirradiated on the predetermined XY flat surface; and accordingly, acalibration coefficient ax,y of the dose monitoring device 26corresponding to other irradiation position (x,y) of the irradiationsubject of the particle beam can be obtained by the calculation unit 44in a similar way.

Further, by the calculation unit 44, the calibration coefficient ao,o ofthe dose monitoring device 26 in which the irradiation position of theirradiation subject corresponds to the position (0,0) is set as areference, and a correction coefficient Ax,y corresponding to otherirradiation position (x,y) of the irradiation subject is found as aratio of the calibration coefficient ao,o with respect to a referenceposition (0,0) as follows:Ax,y=ax,y/ao,o

In this way, the correction coefficient Ax,y of the dose monitoringdevice 26 corresponding to for each irradiation position of theirradiation subject of the particle beam, the correction coefficientAx,y being found by the calculation unit 44, is stored in a database 45.

On the other hand, plans for patients are made by a treatment planningsystem 47. In the plans, an irradiation position and a given dose of anirradiation subject of each spot is designated. Among the designation,as for the given dose for each irradiation position, a correction unit48 performs sensitivity correction in accordance with the correctioncoefficient for each irradiation position of the database 45. In thecase of actual treatment, a dose in which the sensitivity correction isperformed for each irradiation position is sent as irradiation pre-set;and irradiation of the particle beam is performed based on theirradiation pre-set. An irradiation pre-set value and a valuecorresponding to the dose measured by the dose monitoring device 26 arecompared by the counter 42; and when the value corresponding to the doseis the irradiation pre-set value, an accelerator 14 is controlled andtreatment irradiation is completed.

Embodiment 5

FIG. 8 is a block diagram showing a particle beam therapy system using asensitivity correction method for a dose monitoring device in Embodiment5. In Embodiment 5, as in Embodiment 4, plans for patients are made by atreatment planning system 47. In the plans, an irradiation position anda given dose of an irradiation subject of each spot is designated. Amongthe designation, as for the given dose for each irradiation position, acorrection unit 48 performs sensitivity correction in accordance with acorrection coefficient for each irradiation position of a database 45.In the case of actual treatment, a dose in which the sensitivitycorrection is performed for each irradiation position is sent asirradiation pre-set; and irradiation of the particle beam is performedbased on the irradiation pre-set. At this time, a position monitor 11measures the irradiation position of the irradiation subject and inputsmeasured irradiation position (x,y) data to a discrepancy detector 50.On the other hand, irradiation position (x,y) data of the irradiationsubject of each spot made by the treatment planning system 47 isinputted to the discrepancy detector 50. When the discrepancy betweenboth inputs exceeds a predetermined set value, discrepancy informationis sent. In this way, the discrepancy of the irradiation position of theparticle beam may be detected.

Embodiment 6

In Embodiment 1, the description has been made on the case where thesmall parallel flat plate type ionization chamber is located in thelower stream of the dose monitoring device; however, the small parallelflat plate type ionization chamber may be located in an upper stream ofthe dose monitoring device, as depicted, for example, in FIG. 9. In thiscase, a dose of a particle beam 35 is measured by the small parallelflat plate type ionization chamber 38 in a state where a beam positionbefore being scanned by scanning electromagnets is not fluctuated from abeam axis or in a state where, after being scanned, a fluctuation widthbefore reaching the dose monitoring device 26 is smaller than a positionof the dose monitoring device 26; and therefore, the small parallel flatplate type ionization chamber 38 can be naturally smaller than the dosemonitoring device 26 and the ionization chamber 38 does not need to bemoved according to the beam position on the XY flat surfaceperpendicular to the beam axis. Therefore, the small parallel flat platetype ionization chamber 38 can be used being fixed on the beam axis. Inthe case of Embodiment 6, the small parallel flat plate type ionizationchamber 38 is not present at an irradiation position (isocenter) to adiseased part; and therefore, there is an advantage in that the smallparallel flat plate type ionization chamber 38 does not disturb theirradiation and does not need to be removed from a particle beam lineafter sensitivity correction.

Various modifications and alternations of this invention can be achievedto those skilled in the art without departing from the scope and spiritof this invention, and it should be understood that this is not limitedto the respective illustrative embodiments set forth in the description.

The invention claimed is:
 1. A sensitivity correction method for a dosemonitoring device included in a particle beam therapy system which scansa particle beam and irradiates the particle beam to an irradiationposition of an irradiation subject, the sensitivity correction methodcomprising the steps of: measuring, by said dose monitoring device, thedose of the particle beam passing through said dose monitoring device;measuring, by a small ionization chamber that is smaller in size thansaid dose monitoring device, the dose of the particle beam that passesthrough said dose monitoring device, wherein said small ionizationchamber is located upstream of said dose monitoring device with respectto the traveling direction of the particle beam; and calculating, by acalculation unit, a correction coefficient of the dose measured by saiddose monitoring device corresponding to the irradiation position basedon the dose of the particle beam measured by said small ionizationchamber, for improving measurement deterioration due to deflection of anelectrode of the dose monitoring device.
 2. The sensitivity correctionmethod for the dose monitoring device according to claim 1, wherein saidparticle beam therapy system further includes a treatment planningsystem, the method further comprising: scanning the particle beam andirradiating the particle beam to the irradiation position of theirradiation subject, on the basis of the irradiation position of theirradiation subject planned by said treatment planning system; andcalculating, the correction coefficient of the dose measured by saiddose monitoring device corresponding to the irradiation position.
 3. Thesensitivity correction method for the dose monitoring device accordingto claim 1, wherein said particle beam therapy system further includes atreatment planning system, the method further comprising: scanning theparticle beam and irradiating the particle beam to the irradiationposition of the irradiation subject, on a basis of the irradiationposition to be set based on energy of the particle beam and the amountof excitation of X,Y direction scanning electromagnets commanded by saidtreatment planning system; and calculating the correction coefficient ofthe dose measured by said dose monitoring device corresponding to theirradiation position.
 4. The sensitivity correction method for the dosemonitoring device according to claim 1, wherein said particle beamtherapy system further includes a position monitor of the particle beam,the method further comprising: measuring the irradiation position of theparticle beam irradiated to said position monitor; and confirming theirradiation position of the irradiation subject of the particle beam. 5.The sensitivity correction method for the dose monitoring deviceaccording to claim 1, wherein said small ionization chamber is aparallel flat plate type.
 6. The sensitivity correction method for thedose monitoring device according to claim 1, wherein the correctioncoefficient of said dose monitoring device in the case where theirradiation position of the irradiation subject of the particle beam ison an axis of a traveling direction of the particle beam is set as astandard, and the correction coefficient of the dose monitoring devicein the case where the irradiation position of the irradiation subject ofthe particle beam is different from the axis is found by a ratio withthe calibration coefficient is set as the standard.
 7. The sensitivitycorrection method for the dose monitoring device according to claim 6,wherein said small ionization chamber includes an isocenter and isdisposed on a flat surface perpendicular to the axis of the travelingdirection of the particle beam.
 8. A particle beam therapy system whichscans a particle beam and irradiates the particle beam to an irradiationposition of an irradiation subject, said particle beam therapy systemcomprising: a dose monitoring device which measures a dose of theparticle beam; an ionization chamber that is smaller in size than saiddose monitoring device, wherein said ionization chamber measures thedose of a particle beam passing through said dose monitoring device andis located upstream of said dose monitoring device with respect to thetraveling direction of the particle beam; and a calculation unit whichcalculates a correction coefficient of the dose measured by said dosemonitoring device corresponding to the irradiation position based on (i)the dose of the particle beam measured by said dose monitoring device,(ii) the irradiation position, and (iii) the dose of the particle beampassing through said dose monitoring device measured by said ionizationchamber, wherein the irradiation dose is adjusted based on thecorrection coefficient, thereby improving measurement deterioration dueto deflection of an electrode of the dose monitoring device.
 9. Theparticle beam therapy system according to claim 8, further comprising atreatment planning system, wherein on a basis of the irradiationposition of the irradiation subject planned by said treatment planningsystem, the particle beam is scanned and the particle beam is irradiatedto the irradiation position of the irradiation subject; and thecorrection coefficient of the dose measured by said dose monitoringdevice corresponding to the irradiation position is found.
 10. Theparticle beam therapy system according to claim 8, further comprising atreatment planning system, wherein on a basis of the irradiationposition to be set based on energy of the particle beam and the amountof excitation of X,Y direction scanning electromagnets commanded by saidtreatment planning system, the particle beam is scanned and the particlebeam is irradiated to the irradiation position of the irradiationsubject; and the correction coefficient of the dose measured by saiddose monitoring device corresponding to the irradiation position iscalculated.
 11. The particle beam therapy system according to claim 8,further comprising a position monitor of the particle beam, wherein theirradiation position of the particle beam irradiated to said positionmonitor is measured; and the irradiation position of the irradiationsubject of the particle beam is confirmed.
 12. The particle beam therapysystem according to claim 8, wherein said ionization chamber is aparallel flat plate type.
 13. The particle beam therapy system accordingto claim 8, wherein the correction coefficient of said dose monitoringdevice in the case where the irradiation position of the irradiationsubject of the particle beam is on an axis of a traveling direction ofthe particle beam is set as a standard, and the correction coefficientof the dose monitoring device in the case where the irradiation positionof the irradiation subject of the particle beam is different from theaxis is found by a ratio with the calibration coefficient set as thestandard.
 14. The particle beam therapy system according to claim 13,wherein said ionization chamber includes an isocenter and is disposed ona flat surface perpendicular to the axis of the traveling direction ofthe particle beam.
 15. The sensitivity correction method for the dosemonitoring device according to claim 1, wherein said particle beamtherapy system further includes a position monitor positioned proximateto said dose monitoring device, the method further comprises: measuringthe irradiation position of the particle beam irradiated to saidposition monitor.
 16. The particle beam therapy system according toclaim 8, further comprising a position monitor positioned in closeproximity to said dose monitoring device, wherein said position monitoris configured to measure the irradiation position of the particle beamirradiated to said position monitor.
 17. The sensitivity correctionmethod for the dose monitoring device according to claim 1, furthercomprising: converting the dose, measured by the dose monitoring device,to a count value corresponding to the irradiation position of theirradiation subject; and converting, by said small ionization chamber,the measured dose of the particle beam to electric charge correspondingto the irradiation position of the irradiation subject.
 18. The particlebeam therapy system according to claim 8, wherein the dose monitoringdevice is further configured to convert the dose, measured by the dosemonitoring device, to a count value corresponding to the irradiationposition of the irradiation subject; and the ionization chamber isfurther configured to convert the dose, measured by said ionizationchamber, to electric charge corresponding to the irradiation position ofthe irradiation subject.